First make sure you have the virtualenv activated.
Then, that the version of python installed in that virtualenv, is version 3.x and not 2.7 or 2.x.
To check it, simply activate the virtualenv and enter in the shell:
This will give you the installed version, and if it is not correct, you can generate a new virtualenv or install the correct version.
This would be the default option.
If you want to install both versions of python, simply, when calling the command ./manage.py, do it in the following way:
'python3 ./manage.py shell'
Finally, we recommend you to install ipython, which is very useful when you work in the python or django shell.